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The 21 Ω that is not your electrodeMust your TT rod be under 21 ohms?
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A number people misread constantly. On-Site Guide · p.14:
For supplies up to 100 A, 21 Ω is the usual stated maximum resistance of the DNO's earth electrode at their transformer — their star point, on the supply side. It is not a limit on the rod in your customer's garden.
| Figure | What it actually limits |
|---|---|
| 21 Ω | The DNO's electrode at the transformer |
| Up to 200 Ω | Your TT electrode reading — anything from near zero upward |
| 0.8 Ω / 0.35 Ω | Maximum Ze for TN-S and TN-C-S — different systems entirely |
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